Research on the causative agent of Chagas' disease in man, Trypanosoma cruzi, has been conducted primarily with bloodstream, tissue and culture forms of the parasite. A void exists in our knowledge of the infective stage of T. cruzi, the metacyclic which develops in the invertebrate vector and in culture. Techniques for rearing triatomid bugs in the laboratory and for the in vitro cultivation of metacyclic stages now permit comparative investigations of the biology and immunology of these forms. This project proposes to: 1) characterize the growth periods of in vivo infection and in vitro cultivation of these forms, 2) compare the size and morphology of in vivo and in vitro grown metacyclic forms, 3) describe the ultrastructural architecture of metacyclics from triatomes and culture, 4) quantitate the infectivity of in vivo and in vitro produced metacyclics, 5) define the antigenic similarities and/or differences between metacyclic trypomastigotes, culture form epimastigotes and bloodstream form trypomastigotes, and 6) determine the degree of protective immunity elicited by immunization with in vitro produced metacyclic forms to challenge inoculations with in vivo and in vitro grown metacyclics. These studies should provide information about T. cruzi infection in the triatome, the growth and development of metacyclics in vivo and in vitro, biological and serological similarities and/or differences in infective forms from the two sources and insights into resistance to the invasive metacyclic stage of T. cruzi.